Chalcogenide switching devices are used, for example, with phase-change memory devices as an access element. The phase-change non-volatile memory devices are beneficial in applications where data must be retained when power is disconnected. Applications include general memory cards, consumer electronics (e.g., digital camera memory), automotive (e.g., electronic odometers), and industrial applications (e.g., electronic valve parameter storage). The non-volatile memories may use phase-change memory materials, i.e., materials that can be switched between a generally amorphous and a generally crystalline state, for electronic memory applications. The memory of such devices typically comprises an array of memory elements, each element defining a discrete memory location and having a volume of phase-change memory material associated with it. The structure of each memory element typically comprises a phase-change material, one or more electrodes, and one or more dielectrics.
One type of memory element originally developed by Energy Conversion Devices, Inc. utilizes a phase-change material that can be, in one application, switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. These different structural states have different values of resistivity, and therefore each state can be determined by electrical sensing. Typical phase-change materials suitable for memory application include those incorporating one or more chalcogen or pnictogen elements. Unlike certain known devices, these electrical memory devices typically do not use field-effect transistor devices as the memory storage element. Rather, they comprise, in the electrical context, a monolithic body of thin film chalcogenide material. As a result, very little area is required to store a bit of information, thereby providing for inherently high-density memory chips.
Ovonic unified or phase-change memories are an emerging type of electrically-alterable non-volatile semiconductor memories. These memories exploit the properties of materials (phase-change materials) that can be reversibly switched between two or more structural states that vary in the relative proportions of amorphous and crystalline phase regions when subjected to heat or other forms of energy. The term “amorphous” refers to a condition which is relatively structurally less ordered or more disordered than a single crystal and has a detectable characteristic, such as high electrical resistivity. The term “crystalline” as used herein refers to a condition which is relatively structurally more ordered than amorphous and has at least one detectably different characteristic, such as a lower electrical resistivity.
The distinct structural states of a phase-change material exhibit different electrical characteristics, such as resistivity, that can be used to distinguish the different states. Memory or logic functionality is achieved by associating a different memory or logic value with each structural state. Programming occurs by providing the energy needed to stabilize the structural state of the phase-change material associated with the input memory or logic data.
Typically, a memory array includes a matrix of phase-change memory cells, arranged in rows and columns with associated word lines and bit lines, respectively. Each memory cell typically consists of a phase-change storage element connected in series to an access element, where each memory cell is connected between a particular word line and a particular bit line of the array. Each memory cell can be programmed to a particular memory state by selecting the word line and bit line associated with the memory cell and providing a suitable energy pulse across the memory cell. The energy pulse is typically a current pulse applied to the memory cell by supplying a voltage potential between the word line and bit line of the cell. The voltage potential activates the access element connected to the memory element, thereby enabling the flow of current through the memory element. Typical access elements include diodes and transistors. Reading of the memory state is accomplished by similarly selecting the word line and bit line of the memory cell and measuring the resistance (or a proxy therefor (such as the voltage drop across the cell). In order to maintain the state of the memory cell during read, it is necessary to maintain the energy of the read signal at a level below that needed to transform the memory cell from its existing state to a different state.
In addition to memory elements, switching elements, particularly fast switching devices, are desirable for a number of applications. Fast switching elements are capable of being switched between a relatively resistive state and a relatively conductive state and are useful as voltage clamping devices, surge suppression devices, and signal routing devices. Fast switching elements can also be used as access devices in memory arrays.
An important class of fast switching materials are the Ovonic Threshold Switch (“OTS”) materials. OTS materials, like many phase-change memory materials, typically include one or more chalcogen elements. Unlike phase-change memory materials, however, the compositions of OTS materials are such that no change in structural state occurs within the range of normal operation of the material. Instead, the OTS material retains an overall predominantly amorphous structure during operation. Application of a suitable energy signal, typically an electrical energy signal having a voltage above a critical threshold level, induces a change of the electrical characteristics of the OTS device from a relatively resistive quiescent off state to a relatively conductive on state. The relatively conductive state persists for so long as the current passing through the OTS material remains above a critical holding level. Once the energy signal is removed or the current otherwise decreases below the level needed to sustain the relatively conductive on state, the OTS material relaxes back to its relatively resistive quiescent off state.
Under one theory of operation, an OTS material achieves its conductive on state through the formation of a localized, conductive filamentary region that extends across the material between opposing electrical contacts when the voltage applied between the contacts is at or above a threshold voltage. When the current across the material is decreased to below the holding level needed to sustain the conductive state, the filamentary region collapses and the material switches back to its resistive quiescent state. As the material is switched between its resistive and conductive states over multiple cycles of operation, the filamentary region is repeatedly formed and extinguished.
A consequence of this mechanism of operation is that the reproducibility and stability of the switching event over multiple cycles of operation depends on the consistency of the characteristics of the localized filament. Optimal performance requires consistent physical placement of the filament within the OTS material and a reproducible threshold voltage to insure control over initiation of the switching event. It is also necessary for the holding current to remain stable over multiple cycles of operation. In practice, it has been observed that the threshold voltage, holding current, and/or physical placement of the filamentary region of OTS materials may vary upon cycling and the switching characteristics of OTS materials is accordingly compromised.
One strategy for stabilizing the operation of OTS materials is to enlarge the lateral dimensions of the material in the switching device and use large area contacts. This approach tolerates variations in the physical location of the filament upon cycling by insuring the existence of an adequate voltage for switching over a range of positions within the material. As a result, any variability in the stabilized position of the filament that may occur upon repeated cycling is accommodated and the possibility of a failure to switch is minimized. The drawback to this approach, however, is that large area contacts increase the energy required to sustain the off state by facilitating leakage of current through non-switched OTS devices leading to high standby current in product applications. Large area devices are further contrary to the general goal of increasing device density in order to reduce cost.
There is accordingly a need for switching devices that utilize small area contacts (to reduce the energy required to sustain the off state), while providing for consistent switching performance over multiple cycles of operation.